To smooth the movements of a piston, make leaks hard to occur, and prevent errors from occurring in a fluid having a standard volume being fed to a flowmeter to be calibrated.

The moving speed of the piston 2 is controlled in such a way that the differential pressure between the pressure of the fluid on the upstream side and the pressure of the fluid on the downstream side may be zero while the piston 2 passes through at least a measuring section A-B in a cylinder 1. The piston 2 thereby exhibits no resistance (noload) to the fluid and smoothly moves in the cylinder 1 to make leaks hard to occur and prevent errors from occurring in the fluid having a standard volume being fed to the flowmeter 10 to be calibrated.